This invention relates generally to scanning electron beam systems and, more particularly, the apparatus for selectively collecting low-loss electrons backscattered from a specimen for forming low-loss electron images in such systems.
In a scanning electron beam system, a primary electron beam is directed at a particular point on the surface of a specimen in a vacuum. The electron beam penetrates the surface to a depth determined by the beam energy and the angle of incidence between the axis of the beam and the surface. As the beam travels along its path in the specimen, electrons are emitted from the specimen. These electrons may be secondary electrons ejected from the outer shells of atoms which have been excited by the primary beam or electrons from the primary beam which have been backscattered from the specimen. A portion of the backscattered electrons from the primary beam has suffered very little loss in energy because the electrons in this portion, herein referred to as low-loss electrons, have penetrated the surface only a short distance before being backscattered. Electron images formed by monitoring the low-loss electrons as the primary beam is scanned in a raster pattern over the surface are very useful in analyzing the topography and texture of the surface.
In order to obtain the most detailed images, it is required to selectively collect the low-loss electrons having energies within a narrow energy range. Heretofore, apparatus for collecting low-loss electrons included one or two retarding grids which were placed in the path of the backscattered electrons between the specimen and an electron detector (such as a scintillator-light pipe-photomultiplier tube arrangement) to filter electrons other than the low-loss electrons. These collector system, in addition to providing relatively limited energy discrimination, provided poor transmission of the low-loss electrons due to the presence of retarding and accelerating fields on either side of the grids. Signal losses and poor signal-to-noise ratios resulted from extraneous collisions and deflections out of the solid angle of the collector system. The effect of the poor signal-to-noise ratio may be partially overcome by use of accelerating potentials of over 20 KeV for the primary beam; however, this large accelerating potential results in damage to the specimen. In addition, the scintillators used in prior collector systems were not thermally stable and were subject to degradation from bombardment of the high-energy electrons.